Support using mysql as a new storage engine for etcd.

[ngaut]

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

[k8s-ci-robot]

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[k8s-ci-robot]

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[hpatro]

@ngaut Interesting PR.

Curious to understand, why mysql and how does it fare against existing raft implementation in terms of performance.
Also, does RDBMS like mysql provide the same guarantees like raft around serializability of each operation or are there any limitation to the approach ?